Applications known as “search engines” are widely available on computer networks such as intranets and the Internet, including the World Wide Web. Search engines typically enable users to locate desired content, web sites, etc. Users indicate key words of interest, and a search engine responds with search results, and may also provide listings from advertisers. Search engines often charge advertisers to have their listings included in a set of search results. In pay-per-click (PPC) listings, an advertiser only pays the search engine advertising fee when a user selects, e.g., clicks on, the listing. The position of a PPC listing in a set of search results, e.g., second in a set of five listings, may be determined according to a bid amount, i.e., an amount that an advertiser is willing to pay upon a user selection of the listing.
From the standpoint of being selected by a user, it is generally advantageous for a listing to be placed in a higher position, e.g., first as opposed to fourth in a set of five listings, because users often follow a tendency to select listings that are placed higher. Nonetheless, regardless of where they are placed in a set of search results, some listings may be more or less likely to be selected by a user than other listings. Thus, it would be desirable for a search engine to be able to generate additional revenue by providing listings that are more likely than other listings to be selected by a user.